Photonically-Sampled Electronically-Quantized Analog-to-Digital Converter

ABSTRACT

A photonically-sampled electronically-quantized analog-to-digital converter generates an optical signal comprising a series of optical pulses. The optical signal is split into a first and a second optical path. The split optical signal is detected in the first path and then the detected optical signal is converted to a reference digital signal. The split optical signal in the second path is modulated with an input RF signal and a plurality of demultiplexed RF-modulated optically-sampled signals is generated from the modulated optical signal. The plurality of demultiplexed RF-modulated optically-sampled signals is then pulse broadened, detected, and converted to a plurality of sampled-RF digital signals. The reference digital signal and the plurality of sampled-RF digital signals are digital signal processed to generate a digital representation of the input RF signal.

The section headings used herein are for organizational purposes onlyand should not be construed as limiting the subject matter described inthe present application in any way.

Introduction

Analog-to-digital conversion (ADC) is an important and widely-usedelectronic system function that transforms analog, or time-continuous,signals into digital, or discrete-time signals that are most commonlybinary signals including 1's and 0's. This crucial function allows theresulting digitized signals to be further processed by a wide variety oflow-cost and/or sophisticated processing only available with digitalsignal processing electronics. The ADC process consists of two mainsteps: sampling and quantization. Sampling obtains the value of thewaveform at a particular instant in time; quantization determines thedigital, typically binary, representation of a sample. The fidelity ofthe digital representation of an analog signal is related to severalimportant performance parameters of the analog-to-digital conversionsampling and quantization system including bandwidth, speed, time andamplitude jitter, and noise. Many of these parameters are limited by thecapability of the electronic sampling circuits.

Photonic sampling has been developed to eliminate some of thebottlenecks of electronic sampling. One feature of photonic sampling isthat it can provide substantially lower timing jitter of when the sampleis taken. See, for example, A. H. Nejadmalayeri, et. al,. “A 16-fsaperture jitter photonic ADC: 7.0 ENOB at 40 GHz”, Proc. Conf. on Lasersand Electro-optics (CLEO), 2011, paper CThI4. Another feature ofphotonic sampling is that in some architectures, photonic sampling canincrease the bandwidth and reduce the noise of the analog-to-digitalconversion.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teaching, in accordance with preferred and exemplaryembodiments, together with further advantages thereof, is moreparticularly described in the following detailed description, taken inconjunction with the accompanying drawings. The skilled person in theart will understand that the drawings, described below, are forillustration purposes only. The drawings are not necessarily to scale,emphasis instead generally being placed upon illustrating principles ofthe teaching. The drawings are not intended to limit the scope of theApplicant's teaching in any way.

FIG. 1 illustrates a schematic of a prior art photonic analog-to-digitalconverter architecture that has one bank of electronic,analog-to-digital converters on the output of the modulator.

FIG. 2 illustrates a known Mach-Zehnder modulator balanced detectionsystem.

FIG. 3 illustrates a known photonic analog-to-digital converter thatincludes a complementary output modulator feeding a balanced outputconfiguration that includes two banks of electronic analog-to-digitalconverters, one on each complementary output of the modulator.

FIG. 4 illustrates a known plot of the response roll-off of a knownphotonic analog-to-digital converter as a function of pulse width for a20-GHz sine wave input.

FIG. 5 illustrates a known plot of the Effective Number of Bits (ENOB)of a known photonic analog-to-digital converter as a function of pulsewidth.

FIG. 6 illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter system comprising asingle-channel normalization scheme according to the present teaching.

FIG. 7A illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter system with passivedetector-to-ADC interface according to the present teaching.

FIG. 7B illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter system withsubtraction before the ADC according to the present teaching.

FIG. 7C illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter system with passivedetector-to-ADC interfaces and subtraction before the ADCs, as well assingle-channel normalization according to the present teaching.

FIG. 8 illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter that useswavelength interleaving and routing (WIR), as well as single-channelnormalization, passive detector-to-ADC interfaces and subtraction beforethe ADCs according to the present teaching.

FIG. 9 illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter of the presentteaching comprising post-modulator dispersion.

DESCRIPTION OF VARIOUS EMBODIMENTS

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the teaching. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

It should be understood that the individual steps of the methods of thepresent teachings may be performed in any order and/or simultaneously aslong as the teaching remains operable. Furthermore, it should beunderstood that the apparatus and methods of the present teachings caninclude any number or all of the described embodiments as long as theteaching remains operable.

The present teaching will now be described in more detail with referenceto exemplary embodiments thereof as shown in the accompanying drawings.While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art. Those of ordinary skill inthe art having access to the teaching herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein.

The present teaching describes a method and apparatus forphotonically-sampled, electrically-quantized analog-to-digitalconverters that in various embodiments can have any combination of fivefeatures that improve performance and have relatively low cost andcomplexity as well as relatively low power consumption. These featuresinclude single channel normalization, passive detector-to-ADCinterfacing, subtraction prior to the analog-to-digital conversion,post-modulator dispersion, and low noise figure analog-to-digitalconversion.

FIG. 1 is a schematic of a prior art photonic analog-to-digitalconverter architecture 100. A pulsed laser 102 provides a high-speed,low-jitter pulse train to the modulator 104. Pulse trains as describedherein can refer to pulses that are spaced periodically, i.e. uniformly,in time. Pulse trains as described herein can also refer to pulsesspaced arbitrarily in time, i.e. such that the time between pulses isvariable. The output of the modulator 104 is a train of pulsesrepresenting a sampled version of the electrical input signal that is tobe converted to a digital representation. These pulses are sequentiallysent to an optical pulse demultiplexer 106. A typical prior artimplementation of the optical pulse demultiplexer 106 requires amassive, 1×N, optical switch. At present the only practical way toimplement optical switches with the required speed is by using a binarycascade of 1×2 switches. Thus if N=32, realizing a 1×32 switch wouldrequire 31 optical switches, which would require a custom,integrated-optic assembly. This represents a major impediment torealizing this approach when the cost of fabricating such custom switcharrays is combined with the switch drive circuitry that needs to besynchronized with the sample pulses.

A plurality of optical detectors 108, 108′ is optically coupled to theoutput of the optical pulse demultiplexer 106. Each of the plurality ofdetectors 108, 108′ is electrically connected to one of a plurality ofelectronic receivers 110, 110′ which contain active electronic devicesand/or circuits that amplify, provide impedance transformation, etc. Theelectronic receivers provide the interface between the output of thedetectors 108, 108′ and the input to the ADC 112, 112′. In addition tothe cost of the high-speed digital amplifiers in the electronicreceivers, such amplifiers can consume a considerable amount of power.Each of the plurality of electronic receivers 110 is then electronicallyconnected to one of a plurality of electronic analog-to-digitalconverters 112, 112′ so that each electronic analog-to-digital converteronly sees pulses at its electronic sampling rate. The electronicsampling rate of each analog-to-digital converter is many times slowerthan the optical sampling rate. A digital back end process unit 114 isthen used for various processing tasks.

FIG. 1 illustrates a prior art photonic analog-to-digital convertersthat uses short optical pulses to sample electrical signal applied tothe optical modulator. The pulses are converted from optical toelectrical pulses to enable sampling by the electronic analog-to-digitalconverters 112, 112′ using a detector 108, 108′ followed by theelectronic receiver circuits 110, 110′. The sampling function works bestwith short pulses for reasons that will be discussed in conjunction withFIG. 4.

FIG. 2 illustrates a known Mach-Zehnder modulator balanced detectionsystem 200. The balanced detection system 200 includes Mach-Zehndermodulator 202 having an RF input and an optical input that is coupled tothe output of a laser 204. The Mach-Zehnder modulator 202 outputs shownin FIG. 2 are complementary, i.e. they are, at least ideally, of equalamplitudes that are 180° out of phase with respect to one another. Thetwo arms of the Mach-Zehnder modulator 202 are optically coupled to abalanced detector 206 that includes a first and second detector 208, 210in a balanced configuration having an output that is electricallyconnected to an input of a receiver 212.

In operation, an input signal causes a so-called positive-polaritysignal in the first detector 208 and a so-called negative-polaritysignal in the second detector 210. Noise at the input, however, appearsas common-mode noise in the two outputs. That is the noise has the samepolarity in both outputs. Thus, when the first and second detector 208,210 outputs are subtracted, the noise is cancelled and the signal isdoubled as compared to the output of one of the first and the seconddetectors 208, 210. The details of the balanced detection system 200 aredescribed in E. I. Ackerman, et al, “Signal-to-noise performance of twoanalog photonic links using different noise reduction techniques,” 2007International Microwave Symposium Conference Digest, pp. 51-54, Jun.3-8, 2007.

FIG. 3 illustrates a known photonic analog-to-digital converter 300 thatincludes a balanced output configuration that includes a bank of Nelectronic analog-to-digital converters on each complementary-output ofthe modulator 302. The photonic analog-to-digital converter 300 issimilar to the photonic analog-to-digital converter 100 that wasdescribed in connection with FIG. 1. However, the photonicanalog-to-digital converter 300 includes a complementary output opticalmodulator 302 such as the Mach-Zehnder modulator 202 that was describedin connection with FIG. 2.

A pulsed laser 304 provides a high-speed, low-jitter pulse train to theinput of the complementary output optical modulator 302. Complementarytrains of pulses representing a sampled version of the electrical inputsignal propagate on each of a first 306 and second arm 308 of the outputof the modulator 302. These pulses are sequentially sent to a firstoptical pulse demultiplexer 310 and second optical pulse demultiplexer312. Hence this approach requires two of the massive, optical switcharrays that were described in conjunction with FIG. 1. A first andsecond plurality of optical detectors 314, 314′ 316, 316′ are opticallycoupled to the output of the first and second optical pulsedemultiplexers 310, 312. Each of the plurality of detectors in the firstand second plurality of optical detectors 314, 314′, 316, 316′ iselectrically connected to one of a plurality of the first and secondplurality of electronic receivers 318, 318′, 320, 320′. Each of theplurality of electronic receivers in the first and second plurality ofelectronic receivers 318, 318′, 320, 320′ is then electronicallyconnected to one of a plurality of electronic analog-to-digitalconverter in the first and the second plurality of analog-to-digitalconverters 322, 322′, 324, 324′ so that each electronicanalog-to-digital converter (ADC) only sees pulses at its electronicsampling rate. A digital back end process unit 326 is then used forvarious processing tasks.

The subtraction of the complementary outputs is done digitally withsignal processing hardware after the analog-to-digital conversionprocess. This configuration has been used on prior art photonicanalog-to-digital converter demonstrations. See, for example, P. W.Juodawlkis, et al, “Optically sampled analog-to-digital converters,”IEEE Trans. Microwave Theory Tech., vol. 49, pp. 1840-1853, 2001. Seealso reference, S. J. Spector, et al, “Integrated Optical Components inSilicon for High Speed Analog to Digital Conversion,” Proc. SPIE, vol.6477, pp. 647700-1-647700-14, 2007.

The subtraction of complementary outputs substantially improves thesignal integrity of the digital representation of the analog signal. Inaddition to noise cancellation, the full-complementary configurationshown in FIG. 3 allows recovery of the input intensity by summing theoutputs from the two complementary analog-to-digital converterscorresponding to the same pulse on each output. This enables morecomplex linearization and normalization methods, such as the arcsinelinearization of the Mach-Zehnder modulator, and cancellation of AMnoise sidebands on large signals when they come from optical intensitynoise. See, for example, J. C. Twichell and R. Helkey, “Phase-encodedoptical sampling for analog-to-digital converters,” IEEE Photon.Technol. Lett., vol. 12, pp. 1237-1239, September 2000. However, theseadvantages come at the expense of doubling the number of electronic ADCsrequired, which in turn increases the size, weight, power and/or cost ofthe overall unit.

One feature of the photonically-sampled, electrically-quantizedanalog-to-digital converters of the present teaching is optimizing forboth less roll-off and high detection efficiency. FIG. 4 illustrates aplot 400 of the response roll-off of a prior art photonicanalog-to-digital converter as a function of pulse width for a 20-GHzsine wave input. Short pulses are favored because they experience lessroll-off. Reducing the roll-off becomes more important as the analoginput frequency increases. On the other hand, detection is moreefficient with longer pulses. This is because of two factors. First,short pulses can result in high peak power which causes the detector tobecome nonlinear. Second, the timing jitter of the electronicanalog-to-digital converter sampler leads to increased noise when theinput signal to the electronic analog-to-digital converter is varyingrapidly.

One feature of the photonically-sampled, electrically-quantizedanalog-to-digital converters of the present teaching is optimizing forboth high bandwidth and high Effective Number of Bits (ENOB). FIG. 5illustrates a plot 500 of the ENOB of a prior art photonicanalog-to-digital converter as a function of pulse width. The effectivenumber of bits is a measure of the signal-to-noise and distortion ratioof an analog-to-digital converter. The plot in FIG. 5 shows how theeffective number of bits increases with increasing pulse width for asimple low-pass-filter circuit following the detector. A highereffective number of bits is desirable. Thus from the point of view ofincreasing ENOB longer pulses are preferred, which is counter to theneed for short pulses to increase bandwidth. The present teachingdescribes a system and method that resolves these opposing constraints.

Prior art photonic analog-to-digital converters choose either acompromise pulse width that is not ideal for either the sampling or thedetection function, or they use a complex circuit between the detectorand the analog-to-digital converter. See, for example, P. W. Juodawlkis,et al, “Optically sampled analog-to-digital converters,” IEEE Trans.Microwave Theory Tech., vol. 49, pp. 1840-1853, 2001. Generally, using acomplex circuit becomes very difficult to implement as the speed of theelectronic analog-to-digital converter increases. The disadvantages ofprior art photonic analog-to-digital converter technology are the cost,complexity, and the power required for the large amount of digital backend electronics used for sampling, calibration, combining, cancellation,and linearization.

What is needed are analog-to-digital converter systems and methods thatperform more functions in the optical domain as compared to prior artphotonic analog-to-digital converter technology. Furthermore, what isneeded is a photonic analog-to-digital converter system and method thateliminates the challenges associated with handling short-optical-pulsesamples in the electronic domain.

One feature of the photonically-sampled electronically-quantizedanalog-to-digital converter of the present teaching is that it providesa photonic analog-to-digital converter that uses a smaller quantity ofcomponents in a less complex configuration of sampling and quantizingelectronics than known photonic analog-to-digital converters whileachieving substantially the same improvement in signal integrity of thesampled signal by cancelling noise, linearizing the signal, andsuppressing AM sidebands.

FIG. 6 illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter system 600comprising a single-channel normalization scheme according to thepresent teaching. The embodiment illustrated in FIG. 6 includes a laser602, or other optical transmitter, that generates a series of nominallyequal amplitude short optical pulses of light in an optical pulse train604. An optical splitter, or tap 606, splits the optical pulse traininto two or more paths. One path proceeds to an optical modulator 608.The optical modulator 608 can be any type of optical modulator. In someembodiments, the optical modulator 608 is a Mach-Zehnder-type opticalmodulator.

The portion of the optical pulse train 604 that passes through theoptical modulator 608 is modulated with an RF input signal 610 whosedigital representation is desired. The optical modulator 608 imposes anRF input modulation 610 onto the optical pulses of the optical pulsetrain 604 to produce an optically-sampled pulse train 612. TheRF-modulated optically-sampled pulse train 612 is incident on an opticalpulse demultiplexer 614, which splits incoming optical signals intomultiple output ports. One skilled in the art will appreciate thatnumerous types of optical pulse demultiplexer 614 architectures can beused. In some embodiments, the optical demultiplexer 614 splits theincoming optical signal into different output ports based on the timeslot of the optical pulse. Demultiplexed optically-sampled signals 616from each output of the optical pulse demultiplexer 614 are passed torespective detectors 620, 620′ that convert the optical signals intoelectrical signals.

The electrical signals are then passed to electronic receivers 626, 626′that condition the signal, and then to analog-to-digital converters 630,630′, that quantize the electrical analog signals to a digital signals.These signals are then input into N inputs 634, 634′ of a digital signalprocessor 650 comprising a bank of digital back-end electronics andprocessing that provides various functions such as combining,calibration, and linearization. The digital signal processor 650provides a digital output 652. The digital output 652 is a digitalrepresentation of the RF input modulation 610. The digital output 652can be in any one of numerous formats, such as serial or parallel,straight binary or Gray code, high bandwidth, and/or low bandwidth,depending on the particular known digital back end signal processingused.

The portion of the optical pulse train 604 that follows path 640 formsthe single-channel normalization path. The optical pulses in path 640propagate to detector 618. The detected signal is then passed to theelectrical receiver 624 that conditions the signal, and then to areference analog-to-digital converter 628 that converts the electricalanalog signal to a digital signal to produce a reference input at input632 to the digital signal processor 650.

The embodiment of FIG. 6 is referred to herein as a single-channelnormalization. The single-channel normalization architectureadvantageously uses a single detector 618 and a single analog-to-digitalconverter 628 to perform the functions that in prior artimplementations, such as shown in FIG. 3, required a second opticalpulse demultiplexer, 312, as well as a whole bank of N complementarydetectors, 316, N receivers, 320 and N analog-to-digital converters,324.

A feature of the present teaching is that essentially all the opticalintensity noise from the laser is at frequencies that are less than halfthe optical pulse train repetition rate because any higher frequencynoise is aliased back into this frequency range according to the Nyquistsampling theory. Single-channel normalization is thus able to cover theentire frequency range of the input optical intensity noise. Within thisbandwidth, it will perform the full normalization and noise cancellationfunctions. That is, the single-channel normalization scheme willsuppress input optical intensity noise, AM sideband noise due to inputoptical intensity noise, and the reference input 632 will provide thereference signal at a signal level that enables arcsine and otherlinearization algorithms. While single-channel normalization will notsuppress noise at frequencies above half the electronicanalog-to-digital sample rate, which is the bandwidth of thephotonically-sampled, electronically quantized analog-to-digitalconverter, this limitation is usually not significant because mostoptical intensity noise and intensity variation important fornormalization is at low frequency. Differences in channel transmissionthrough the pulse demultiplexer 614 appear as channel gain offsets, notas noise. These differences in channel transmission can be compensatedby routine analog-to-digital converter calibration algorithms.

As compared to the configuration shown in FIG. 3, only about half of thenumber of analog-to-digital converters 630, 630′ and consequently onlyabout half the number of inputs to the digital signal processor isrequired in the single-channel normalization architecture. Theelectronic analog-to-digital converters account for a large fraction ofboth the electrical power consumed and the total system cost forphotonic analog-to-digital converters of the prior art. Thus, theembodiment shown in FIG. 6 of the current teaching advantageouslyprovides a substantial reduction in cost, complexity and electricalpower.

One skilled in the art will appreciate that the single-channelnormalization scheme described herein can be used in combination withany type of photonic analog-to-digital converter architecture. Using thesingle-channel normalization scheme of the present teaching with knownphotonic analog-to-digital converter substantially reduces overall cost,complexity and power consumption, with only a small reduction inperformance.

One feature of some of the photonically-sampled electronically-quantizedanalog-to-digital converter system embodiments of the present teachingis that some embodiments eliminate the need for an active receiverinterface between the photodetector and ADC. FIG. 7A illustrates anembodiment of the photonically-sampled electronically-quantizedanalog-to-digital converter 700 with passive detector-to-ADC interfaceaccording to the present teaching. The laser 702, or other opticalsource, generates an optical signal that is a train of optical pulses704. The laser 702 is connected to an optical modulator 706. The opticalmodulator 706 can be any type of optical modulator. In some embodiments,the optical modulator 706 is a Mach-Zehnder-type optical modulator. AnRF input signal 708 whose digital representation is desired is input tothe modulator 706. The optical modulator 706 imposes the RF inputmodulation 708 onto the optical pulses of the optical pulse train 704 toproduce an optically-sampled pulse train 710.

The RF-modulated optically-sampled pulse train 710 is incident on anoptical pulse demultiplexer 712, which splits incoming optical signalsinto multiple output ports. One skilled in the art will appreciate thatnumerous types of optical pulse demultiplexer 712 architectures can beused. In some embodiments, the optical demultiplexer 712 splits theincoming optical signal into different output ports based on the timeslot of the optical pulse. Demultiplexed optically-sampled signals 714from each output of the optical pulse demultiplexer 712 are passed torespective detectors 716, 716′ that convert the optical signals intoelectrical signals.

The electrical signal output of the detectors 716, 716′ are input to aninterface 718, 718′. The interface 718, 718′ connects to an ADC 720,720′, that quantize the electrical analog signals to digital signals.These signals are then input into N inputs 724, 724′ of a digital signalprocessor 726 comprising a bank of digital back-end electronics andprocessing that provides various functions such as combining,calibration, and linearization. The digital signal processor 726provides a digital output 728. The digital output 728 is a digitalrepresentation of the RF input modulation 708. The digital output 728can be in any one of numerous formats, such as serial or parallel,straight binary or Gray code, high bandwidth, and/or low bandwidth,depending on the particular known digital back end signal processingused.

In some embodiments, the interface 718, 718′ performs passive frequencyresponse shaping and impedance transformation on the respectiveelectrical signal output from the detectors 716, 716′. In various otherembodiments, interface 718, 718′ performs other passive filterfunctions. Interface 718, 718′ contains only passive elements and doesnot contain any active electronic devices and/or circuits that amplify.The reason the amplifier can be eliminated is because the optical pathfrom laser 702 to photodetector 716, 716′ is designed such that thephotodetector 716, 716′ produces sufficient current, when combined withthe sensitivity of the ADC 720, 720′, to drive the ADC 720, 720′ to fullscale. Eliminating the need for post-detector amplification cansignificantly reduce the cost, complexity and power consumption of thephotonically-sampled, electrically quantized ADC 700.

In some embodiments, the interface 718, 718′ between the output of thedetector 716, 716′ and the input to the ADC 720, 720′ is a separatecomponent. In some embodiments, the interface 718, 718′ is integratedwith the detector 716, 716′ output and/or ADC 720, 720′ input. Forexample, in embodiments for which the interface 718, 718′ needs toinclude low pass filtering, then a separately identifiable low passfilter can be included. Alternatively a low pass filter function of theinterface 718, 718′ can be realized using the output resistance of thedetector 716, 716′ output together with the input capacitance of the ADC720, 720′. Similarly, in embodiments for which the interface 718, 718′performs a function that requires inductance, then the inductance of thephotodetector 716, 716′ bond wire alone, or a length of conductor alone,or a combination of both can be used for the interface 718, 718′. Itshould be understood that the interface element that performs onlypassive function according to the present teaching can be used invarious other embodiments that do not include a demultiplexer. It willbe understood by those skilled in the art that these embodiments onlyinclude a single detector and single analog-to-digital converter thatare connected via the passive interface.

One feature of the present teaching is performing subtraction prior to,or at the input to, the analog-to-digital converters. This subtractionprovides, for example, cancellation of input optical intensity noise.FIG. 7B illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter 730 withsubtraction before the ADC according to the present teaching. The laser732, or other optical source, generates an optical signal that is atrain of optical pulses 734.

The output of the laser 732 is optically coupled to an input of anoptical modulator 736 with balanced outputs. The optical modulator 736with balanced or complementary outputs is shown as a Mach-Zehndermodulator 736. The optical modulator 736 imposes an RF input 738 on thetrain of optical pulses 734, and generates optically-sampled signals atcomplementary outputs 735, 737. The complementary outputs 735, 737represent equal and opposite amplitudes, i.e., a so-called positiveoutput 735 and a so-called negative output 737.

The optically-sampled signals generated at the positive complementaryoutput 735 of the optical modulator 736 passes to a first pulsedemultiplexer 740, and the optically-sampled signals generated at thenegative complementary output 737 of the optical modulator 736 passes toa second pulse demultiplexer 740′. The demultiplexers 740, 740′ can useany one of various demultiplexer architectures, such aswavelength-division and time-division demultiplexing. The outputs of thedemultiplexers 740, 740′ generate separate demultiplexedoptically-sampled signals 741, 741′, 742, 742′ from theoptically-sampled signals at the input of the demultiplexers 740, 740′.

The outputs of the demultiplexers 740, 740′ are optically coupled to theoptical detectors 743, 743′, 744, 744′ where they are converted toelectrical signals. The outputs of the optical detectors 743, 743′, 744,744′ are electrically connected to receivers 745, 745′, 746, 746′. Thereceivers 745, 745′, 746, 746′ provide the detected signals to therespective positive and negative inputs of the ADC 747, 747′. In someembodiments, the receivers 745, 745′, 746, 746′ are replaced byinterfaces, such as the interfaces 718, 718′ described in connectionwith FIG. 7A.

Performing subtraction prior to, or at the input to, theanalog-to-digital converters advantageously reduces the number of ADC's474, 474′. The outputs of the receivers 745, 746 are electricallyconnected to positive inputs of analog-to-digital-converters 747, 747′.The outputs of the receivers 745′, 746′ are electrically connected tonegative inputs of analog-to-digital-converters 747, 747′. By performingsubtraction prior to, or at the input to, the analog-to-digitalconverters 747, 747′, the present teaching uses N ADCs 747, 747′. Thisis half the number of analog-to-digital converters of known apparatus,such as the apparatus shown in FIG. 3, which required 2N ADCs. In someembodiments balanced detectors are used to perform the subtractionfunction prior to being interfaced to single-ended analog-to-digitalconverters as an alternative to the use of balanced or differentialinput analog-to-digital converters that is shown in FIG. 7B. Since thetotal power consumed by the photonically-sampled, electronicallyquantized ADC 730 is often dominated by the power consumed by theelectronic ADCs, cutting the number of electronic ADCs in half willsubstantially reduce the overall power consumed by thephotonically-sampled, electronically-quantized ADC of the presentteaching.

The output of the analog-to-digital converters 747, 747′ provide Ninputs 748, 748′, etc. to the digital signal processor 749. The digitalsignal processor 749 provides a digital output 751. The digital output751 is a digital representation of the RF input 738 that can be one ofnumerous types of data formats, including serial or parallel, straightbinary or Gray code, high bandwidth and/or low bandwidth depending onthe particular known digital back end electronics used. It should beunderstood that performing subtraction prior to, or at the input to, theanalog-to-digital converters according to the present teaching can beused in various other embodiments that do not include a demultiplexer.It will be understood by those skilled in the art that these embodimentsonly include a single detector and single analog-to-digital converter inwhich subtraction is performed at the input or prior to the input.

The photonically-sampled electronically-quantized analog-to-digitalconverter 730 shown in FIG. 7B uses complementary detection. Therefore,every channel separately has the input optical intensity noisecancelled. Consequently, the input intensity noise is cancelled over thefull bandwidth of the photonic analog-to-digital converter. The balanceddetectors do not provide AM sideband noise suppression, nor do theyprovide normalization for the arcsine or other linearization algorithms.In some embodiments, these functions are accomplished by asingle-channel normalization up to the bandwidth of a singleanalog-to-digital converter, which is half its sample rate. A keyinsight of the present teaching is that a complementary detectionresults in a level of cancellation that is good enough for mostpractical systems because they are able to tolerate a higher level ofuncancelled AM sideband noise and un-normalized high-frequencyfluctuations than they can tolerate uncancelled baseband opticalintensity noise.

FIG. 7C illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter 750 with passivedetector-to-ADC interface and subtraction before the ADC as well assingle-channel normalization according to the present teaching. Thelaser 752, or other optical source, generates an optical signal that isa train of optical pulses 766. The optical signal is split by a tap orsplitter 754. The first portion of the optical beam from the splitter754 is directed to an input to an optical detector 756. An output of theoptical detector 756 is electrically connected to a passivedetector-to-ADC interface 758. The laser pulse power, optical pathimpairments from the laser to the detector, detector response, and ADCsensitivity are chosen such that no electrical amplification is requiredin the interface 758, and yet the ADC is driven at full scale. In someembodiments, the ADC is driven with sufficient current to produce adrive at greater than half-scale of the ADC. The output of the interface758 is electrically connected to an input of a reference analog todigital converter 760. The output of the reference analog-to-digitalconverter 760 is electrically coupled to an input 778 of the digitalsignal processor 790.

In some embodiments, interface 758 performs passive frequency responseshaping and impedance transformation. In various other embodiments,interface 758 performs other passive filter functions. Interface 758contains only passive elements and does not contain any activeelectronic devices and/or circuits that amplify. The reason theamplifier can be eliminated is because the optical path from laser tophotodetector 756 is designed such that the photodetector 756 producessufficient current, when combined with the sensitivity of the ADC 760,to drive the ADC 760 to full scale. Eliminating the need forpost-detector amplification can significantly reduce the cost,complexity and power consumption of the photonically-sampled,electrically quantized ADC.

In some embodiments, the interface 758 between the output of thedetector 756 and the input to the ADC 760 is a separate component. Insome embodiments, the interface 758 is integrated with the detector 756output and/or ADC 760 input. For example, in embodiments for which theinterface 758 needs to include low pass filtering, then a separatelyidentifiable low pass filter can be included. Alternatively a low passfilter function of the interface 758 can be realized using the outputresistance of the detector 756 output together with the inputcapacitance of the ADC 760. Similarly, in embodiments for which theinterface 758 performs a function that requires inductance, then theinductance of the photodetector 756 bond wire alone, or a length ofconductor alone, or a combination of both can be used for the interface758.

The second output of the splitter 754 is optically coupled to an inputof the optical modulator 764 with balanced outputs. The opticalmodulator 764 with balanced or complementary outputs is shown as aMach-Zehnder modulator 764. The optical modulator 764 imposes an RFinput 762 on the train of optical pulses 766, and generatesoptically-sampled signals 765, 767 at complementary outputs. Theoptically-sampled signals 765, 767 at complementary outputs representequal and opposite amplitudes, i.e., a so-called positive output and aso-called negative output.

The optically-sampled signals 765 generated at the positivecomplementary output of the optical modulator 764 passes to a firstpulse demultiplexer 768, and the optically-sampled signals 767 generatedat the negative complementary output of the optical modulator 764 passesto a second pulse demultiplexer 768′. The demultiplexers 768, 768′ canuse any one of various demultiplexer architectures, such aswavelength-division and time-division demultiplexing. The outputs of thedemultiplexers 768, 768′ generate separate demultiplexedoptically-sampled signals 771, 771′, 781, 781′ from theoptically-sampled signals 765, 767.

The outputs of the demultiplexers 768, 768′ are optically coupled to theoptical detectors 770, 770′, 780, and 780′ where they are converted toelectrical signals. The outputs of the optical detectors 770, 770′, 780,and 780′ are electrically connected to passive detector-to-ADCinterfaces 772, 772′, 782, and 782′. These passive interfaces 772, 772′,782, and 782′ are similar in design to the interface 758 describedabove. The interfaces 772, 772′, 782, and 782′ provide the detectedsignals to the respective ADC 774, 774′. In some applications, theinterfaces 772, 772′, 782, and 782′ can be low-pass filters (LPF), inother applications the interfaces 772, 772′, 782, and 782′ can beband-pass filters (BPF). Also the interfaces 772, 772′, 782, and 782′can be either a separate element as shown in FIG. 7C, or they can beincorporated into one of the existing elements. An example of the lattercase would be to implement a low pass filter function by selecting thefrequency response of the detector to roll off at the desired frequency.

One feature of the present teaching is performing subtraction prior to,or at the input to, the analog-to-digital converters. The outputs of theinterfaces 772, 782 are electrically connected to positive inputs ofanalog-to-digital-converters 774, 774′. The outputs of the interfaces772′, 782′ are electrically connected to negative inputs ofanalog-to-digital-converters 774, 774′. By performing subtraction priorto, or at the input to, the analog-to-digital converters, the presentteaching uses N ADCs 774, 774′. This is half the number ofanalog-to-digital converters of prior art apparatus, such as theapparatus shown in FIG. 3, which required 2N ADCs. In some embodimentsbalanced detectors are used to perform the subtraction function prior tobeing interfaced to single-ended analog-to-digital converters as analternative to the use of balanced or differential inputanalog-to-digital converters that is shown in FIG. 7C. Since the totalpower consumed by the photonically-sampled, electronically quantized ADCis often dominated by the power consumed by the electronic ADCs, cuttingthe number of electronic ADCs in half will substantially reduce theoverall power consumed by the photonically-sampled,electronically-quantized ADC of the present teaching.

The output of the analog-to-digital converters 774, 774′ provide Ninputs 776, 776′, etc. to the digital signal processor 790. The digitalsignal processor 790 provides a digital output 792. The digital output792 is a digital representation of the RF input 762 that can be one ofnumerous types of data formats, including serial or parallel, straightbinary or Gray code, high bandwidth and/or low bandwidth depending onthe particular known digital back end electronics used. The digitalsignal processor 790 uses the output of the reference analog-to-digitalconverter 760 to improve signal integrity of the digital representationof the input RF signal by normalization, linearization, noisecancellation and AM sideband suppression.

The photonically-sampled electronically-quantized analog-to-digitalconverter 750 shown in FIG. 7C uses complementary detection. Therefore,every channel separately has the input optical intensity noisecancelled. Consequently, the input intensity noise is cancelled over thefull bandwidth of the photonic analog-to-digital converter. The balanceddetectors do not provide AM sideband noise suppression, nor do theyprovide normalization for the arcsine or other linearization algorithms.These functions are accomplished by the single-channel normalization, upto the bandwidth of a single analog-to-digital converter, which is halfits sample rate. A key insight of the present teaching is that thisresults in a level of cancellation that is good enough for mostpractical systems because they are able to tolerate a higher level ofuncancelled AM sideband noise and un-normalized high-frequencyfluctuations than they can tolerate uncancelled baseband opticalintensity noise.

FIG. 8 illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter 800 according tothe present teaching that advantageously uses multiple wavelengths toachieve both interleaving and routing (WIR) of the sample pulses. Thephotonically-sampled electronically-quantized analog-to-digitalconverter 800 uses a complementary-output optical modulator, such theMach-Zehnder modulator 814 shown in FIG. 8 with a passivedetector-to-ADC interface and subtraction prior to the ADC, both ofwhich were described in connection with FIG. 7 combined with thesingle-channel normalization described in connection with FIG. 6.

Ideally the sampling waveform would be a train of impulse functions,i.e. pulses of zero width in time. Since no such waveform can begenerated by an actual system, the sampling waveform of the presentteaching approximates this ideal case by sampling with a pulse widththat is less than a particular fraction of a period of the frequency ofthe sampled waveform. In one particular embodiment of the presentteaching, a waveform is sampled with a sampling pulse that is < 1/10 ofthe period of the highest frequency in the waveform to be sampled. Thishas been found to effective.

There are two key design parameters that determine the number of pulsesthat can be interleaved from a single laser pulse: (1) the spectralbandwidth of the laser pulse; and (2) the desired width of the samplingpulse, which is related to the maximum electrical frequency to besampled. Consider, for example, an electrical waveform with a bandwidthof 10 GHz. The period of this waveform is 1/10¹⁰ Hz=10⁻¹⁰ seconds, or100 picoseconds. To effectively sample this waveform would require asampling pulse of width Δt<10 picoseconds. If one assumes that the shapeof the pulses in the sampling waveform is approximately Gaussian, thenit is well known to those skilled in the art that the product of theminimum time and the bandwidth product of a Gaussian pulse, Δt×Δf=0.441.Hence, the bandwidth of the sampling waveform Δf=0.441/10⁻¹¹=44.1 GHz,or equivalently 0.35 nm in wavelength, where we have converted thebandwidth in frequency in Hz to a bandwidth in wavelength in metersusing the well-known relationship Δf/f=Δλ/λ and assuming a nominalcenter wavelength of λ=1.55×10⁻⁶ m. Lasers are presently commerciallyavailable that generate narrow pulses that have an optical bandwidthof >30 nm. Hence using such a laser, it is possible to divide each laserpulse into 30/Δλ=85 separate pulses, each with a different wavelength.Various embodiments will use a number of wavelengths that is based onthe spectral bandwidth of the laser pulse and on the electricalfrequency of the signal to be sampled, as described herein.

A laser 802, or other optical source known in the art, generates anoptical signal including a train of optical pulses where each of thepulses includes multiple wavelengths. The optical signal is split by atap or splitter 804. A first output of the splitter 804, which feeds thesingle-channel normalization channel, is optically coupled to a detector860. An output of the detector 860 is electrically coupled to a passivedetector-to-ADC interface 862. In some embodiments, the interface 862 isa low-pass filter (LPF). An output of the interface 862 is electricallyconnected to an analog input of the reference analog-to-digitalconverter 864. The digital output of the reference analog-to-digitalconverter 864 is electrically connected to an input 866 of the digitalsignal processor 840.

Referring back to FIGS. 6-7C, in some embodiments of thephotonically-sampled, electronically quantized ADC the rate of opticalpulses from the optical source 602, 702 is equal to the rate of opticalpulses in the sampling pulse train 612, 716. In other embodiments, itmay be desirable or necessary to achieve a faster rate from pulse train612, 716 than the rate of pulses from the optical source 602, 702, 732,752. Referring to FIG. 7C, to achieve a faster sampling pulse trainrate, an interleaver can be inserted between the output of the tap 754and the input to the modulator 764. Both time and wavelengthinterleavers can be used in the photonically-sampled,electronically-quantized ADC of the present teaching.

Referring again to FIG. 8, a second output of the splitter 804 isoptically coupled to a wavelength interleaver portion of the wavelengthinterleaver and router (WIR). The wavelength interleaver includes thewavelength division demultiplexer (WDD) 818. The WDD 818 separates eachof the multiple wavelength pulses into separate outputs. The outputs ofthe WDD 818 are optically coupled to a plurality of optical delayelements that can be implemented in a number of ways; FIG. 8 shows usingoptical fibers 870, 872, and 874. Each of the plurality of optical delayelements comprising optical fibers 870, 872, and 874 provides a desiredrelative time delay between each of the different wavelength opticalpulse trains. An output of each of the plurality of optical delayelements comprising optical fibers 870, 872, and 874 is opticallycoupled to a respective input of a wavelength division multiplexer (WDM)822 so that the relative time-delay optical pulse trains with differentwavelengths are recombined in the WDM 822. The WDM 822 generates at anoutput a pulse train consisting of multiple optical pulses each withdifferent wavelengths, each in a different time slot. The resultingoutput of the wavelength interleaver provides an interleaved opticalsampling signal 816. A wavelength interleaver has the additionaladvantage that the wavelength encoding of the sampling pulses means thata wavelength demultiplexer can be used to implement a particularlysimple form of the optical pulse demultiplexer. For an example ofwavelength demultiplexing in a system using photonic ADC, see A. H.Nejadmalayeri, et. al,. “A 16-fs aperture jitter photonic ADC: 7.0 ENOBat 40 GHz”, Proc. Conf. on Lasers and Electro-optics (CLEO), 2011, paperCThI4.

The output of the wavelength interleaver at the output of WDM 822 isoptically coupled to an input of an optical modulator 814 with balancedoutputs, which is shown in FIG. 8 as a Mach-Zehnder modulator. Theoptical sampling signals 816 are then modulated by the modulator 814.The modulator 814 imposes an RF modulation signal from RF input 812 onthe optical sampling signals 816, and generates complementary outputs ofan optically-sampled signal, so-called positive optically-sampled signal824 and so-called negative optically-sampled signal 826. Thesecomplementary outputs represent sampled, equal and opposite amplitudesof the RF signal.

A first output of the optical modulator 814, which generates a positiveoptically-sampled signal 824, is optically coupled to an input of a WDD828, which provides the routing portion of the WIR. The WDD 828generates a plurality of wavelength demultiplexed outputs 832, 832′ ofthe positive optically-sampled signal 824 at a plurality of outputs. Asecond output of the optical modulator 814, which generates a negativeoptically-sampled signal 826, is optically coupled to an input of a WDD830. The WDD 830 generates a plurality of wavelength demultiplexedsignals 834, 834′ of the negative optically-sampled stream 826 at aplurality of outputs. The router part of the WIR uses a readilyavailable component, a WDD, to perform the function of the optical pulsedemultiplexer without the need for a massive, custom optical switch.Such a switch is shown in prior art presented in FIG. 1 element 106 andFIG. 3 element 310. Hence, embodiments of the present teaching includingWIR removes one of the main impediments to a practical realization of aphotonically-sampled, electronically-quantized ADC.

Each of the plurality of outputs of the WDD 828 is optically coupled toan input of a respective one of a plurality of optical detectors 836,836′ that detect a respective one of the plurality of wavelengthdemultiplexed signals 832, 832′. Each electrical output of the pluralityof optical detectors 836, 836′ is electrically connected to an input ofa respective one of a plurality of passive detector-to-ADC interfaces838, 838′. Similarly, each of the plurality of outputs of the WDD 830 isoptically coupled to an input of a respective one of a plurality ofoptical detectors 837, 837′ that detect a respective one of theplurality of wavelength demultiplexed signals 834, 834′. Each electricaloutput of the plurality of optical detectors 837, 837′ is electricallyconnected to an input of a respective one of a plurality of passivedetector-to-ADC interfaces 839, 839′.

Each output of the plurality of interfaces 838, 838′ is electricallyconnected to a positive input of a respective one of a plurality ofanalog-to-digital-converters 850, 850′. Each output of the plurality ofinterfaces 839, 839′ is electrically connected to a negative input of arespective one of a plurality of analog-to-digital-converters 850, 850′.Each of the plurality of analog-to-digital converters 850, 850′ isreceiving one particular sample of the RF signal 812 that was encoded ona particular wavelength. The plurality of analog-to-digital-converters850 generates N outputs 852, 852′, which are electrically connected to Ninputs of digital signal processor 840. The digital signal processor 840provides a digital output 842. The digital output 842 is a digitalrepresentation of the RF modulation signal at the RF input 812, whichcan be one of various known formats, including serial or parallel, highbandwidth and/or low bandwidth, depending on the particular knowndigital signal processing electronics that is used. The digital signalprocessor 840 uses the output of the reference analog-to-digitalconverter 864 to improve signal integrity of the digital representationof the input RF signal by normalization, linearization, noisecancellation and AM sideband suppression.

A key insight of the present teaching is that essentially all theoptical intensity noise from the laser is at frequencies that are lessthan half the pulse repetition rate because any higher frequency noiseis aliased back into this frequency range according to the Nyquistsampling theory. The single-channel normalization is thus able to coverthe entire frequency range of the input optical intensity noise.Differences in channel transmission through the pulse demultiplexer,such as demultiplexer 768 in FIG. 7C or the WIR as shown in FIG. 8,appear as channel gain offsets, not as noise. These differences inchannel transmission can be compensated by routine analog-to-digitalconverter calibration algorithms.

FIG. 9 illustrates an embodiment of the photonically-sampledelectronically-quantized analog-to-digital converter 900 of the presentteaching that includes a dispersive component located after themodulator. The analog-to-digital converter 900 includes an opticalsource, such as a laser 902 that generates a train of optical samplingpulses 904. The output of the laser 902 is optically coupled to an inputof the optical modulator 908. The optical modulator 908 modulates an RFmodulation signal applied to the input 910 onto the sampling pulsesgenerated by the laser 902 to produce the optically sampled signal 912.The optical sampling pulses, and resulting optically sampled signalcomprise short duration optical pulses.

The output of the optical modulator is coupled to an input of theoptical pulse demultiplexer 920 with a dispersive optical element 914.The dispersive optical element 914 increases the width of the pulses.The dispersive optical element can be any dispersive element known inthe art. A fiber Bragg grating can be used to implement the dispersiveoptical element. The dispersive optical element can also be a length ofdispersive optical fiber in which the amount of dispersion in thedispersive optical fiber 914 and the length of the optical dispersivefiber 914 are chosen so as to provide an appropriate pulse length for agiven application and/or to optimize detection. Because short pulseshave large optical bandwidths in some embodiments, only a few hundredmeters of dispersive fiber 914 are required.

The dispersive optical element 914 can be positioned anywhere betweenthe optical modulator 908 and the plurality of detectors 924, 924′. Insome embodiments, the dispersive optical element 914 is positionedimmediately after the optical modulator 908 as shown in FIG. 9. In otherembodiments, the dispersive optical element 914 is positionedimmediately before the plurality of optical detectors 924, 924′. In someembodiments, the total dispersion may be achieved by using multipledispersive optical elements. Continuing the example of using dispersiveoptical fiber to implement the dispersive optical element 914, some ofthe length of the dispersive optical fiber is positioned at one locationin the photonic analog-to-digital converter, and the remainder of thelength of dispersive fiber is positioned at one or more other locationsin the photonic analog-to-digital converter. It should be understoodthat the dispersive element of the present teaching can be used invarious other embodiments that do not include a demultiplexer. It willbe understood by those skilled in the art that these embodiments onlyinclude a single detector and single analog-to-digital converter.

In the embodiment shown in FIG. 9, the output of the dispersive opticalelement 914 is optically coupled to an input of the optical pulsedemultiplexer 920. The optical pulse demultiplexer 920 demultiplexes thesampled, and if preceded by a dispersive optical element, dispersedoptical pulse train 916 into multiple output ports. In some embodiments,the optical demultiplexer 920 splits the incoming optical signal into aplurality of output ports based on the time slot of the optical pulse.Each of the plurality of demultiplexer output ports is optically coupledto a respective one of a plurality of optical detectors 924, 924′. Theoptical detectors 924, 924′ convert the demultiplexed dispersed opticalpulse trains 922, 922′ into an electrical signal. The dispersed opticalpulse trains comprise longer-duration pulses than the short pulses ofoptically sampled signal 912.

The output of each of the plurality of optical detectors 924, 924′ iselectrically connected to an input of a respective one of a plurality ofelectrical receivers 926, 926′ that condition the signals. The output ofeach of the plurality of the electrical receivers 926, 926′ iselectrically connected to the input of a respective one of the pluralityof analog-to-digital converters 930, 930′.

The embodiment illustrated in FIG. 9 shows a bank of electricalreceivers 926, 926′ that connect the detectors 924, 924′ to the ADC 930,930′. Alternatively, other embodiments may utilize a passive interfaceinstead of the receiver 926, 926′ to connect the detector 924, 924′ tothe ADC 930, 930′. The dispersive optical element may be utilized inconjunction with any of the embodiments shown in FIGS. 6, 7 and/or 8.

The plurality of analog-to-digital converters 930, 930′ convert theelectrical analog signals to digital signals. The plurality ofanalog-to-digital-converters 930, 930′ generates N outputs 934,934′,which are electrically connected to N inputs of a digital signalprocessor 950. The digital signal processor 950 provides a digitaloutput 952. The resulting digital output 952 is a digital representationof the RF modulation at input 910, which can be one of various knownformats, including serial or parallel, straight binary or Gray code,high bandwidth and/or low bandwidth, depending on the particular knowndigital back end signal processing used.

One feature of the embodiment shown in FIG. 9 is that it beneficiallyavoids what was heretofore believed to be fundamentally opposingconstraints on the width of the optical sampling pulse that forceddesigners to either compromise on optical pulse width and/or the complexcircuitry required to manage the performance with regard to pulse widthof prior art photonic analog-to-digital converters. Theanalog-to-digital converter of the present teaching performs samplingwith short pulses, but the detection and subsequent processing areconducted on longer, dispersed pulses. As such, the response roll-off ofknown systems as described in connection with FIG. 4 takes onsubstantially the value associated with the sampling, short, pulsedurations. However, detection is efficient using the longer dispersedpulses with lower peak power avoiding detector nonlinearity and lowernoise owing to lower bandwidth pulse leading and trailing edges.Furthermore, the effective number of bits for the embodiment shown inFIG. 9 would take on the substantially the value associated with thelonger pulses in the receive chain of the system, thus the effectivenumber of bits is higher, as described in conjunction with FIG. 5.

As the performance of analog-to-digital converters improves, it hasbecome possible to move the analog-to-digital converter forward in thereceive chain, i.e.

move it closer to the antenna with the ultimate objective to have theanalog-to-digital converter connected directly to the antenna output.One factor that is presently limiting achieving this objective is thenoise figure of prior art analog-to-digital converters, which ispresently limited to about 20 dB. The noise figure of state-of-the-artRF receivers is less than 6 dB. Hence at a minimum, a low noiseamplifier is needed between the antenna and the analog-to-digitalconverter. The present lower bound on the noise figure of prior-art,high-speed, analog-to-digital converters is fundamentally set by acombination of electronic sampling and the flash topology used toimplement them.

Unlike electronic sampling, photonic sampling can provide the necessarygain, with low noise figure, prior to the electronic analog-to-digitalconverter. As is well known in the art, inserting a stage withsufficient low-noise gain before a stage with higher noise figure canreduce the overall noise figure of the gain plus high noise figurestage. One feature of the present teaching is the recognition that thegain that photonic sampling can provide, can make it possible to sampledirectly the output of an antenna and do so with sufficiently low noisefigure to make such an approach competitive with the noise figure thatcan be achieved with a low noise amplifier.

Wide bandwidth photonic links with gain and low noise figure have beendemonstrated; see for example E. I. Ackerman, et al, “Signal-to-noiseperformance of two analog photonic links using different noise reductiontechniques,” 2007 International Microwave Symposium Conference Digest,pp. 51-54, Jun. 3-8, 2007. Prior art system designs fail to realize thatphotonic sampling can be considered to be a type of optical link, butunlike a conventional optical link where both the input and output arecontinuous signals, a photonic sampler performs equivalent functions ofan optical link in which the input is continuous but the output occursonly at discrete instants in time, which are commonly referred to assamples. A photonic sampler with gain and low noise figure inconjunction with an electronic quantizer, i.e. an electronicanalog-to-digital converter, can produce a system with a sufficientlylow noise figure to sample directly the output of an antenna.

Architecture of the electronically-quantized analog-to-digital convertersystem with low noise figure can be similar to the known photonicanalog-to-digital converter architecture of FIG. 1, or to any of the newarchitectures described in connection with FIGS. 6 through 9. A pulsedlaser provides a high-speed, low-jitter pulse train to the modulator.The output of the modulator is a train of pulses representing a sampledversion of the electrical input signal that is to be converted to adigital representation. These pulses are sequentially sent to an opticalpulse demultiplexer. A plurality of optical detectors is opticallycoupled to the output of the optical pulse demultiplexer. Each of theplurality of detectors is electrically connected to one of a pluralityof electronic receivers which typically contain active electronicdevices and/or circuits that amplify, provide impedance transformation.The electronic receivers provide the interface between the output of thedetectors and the input to the electronic ADC. In some embodiments, thereceivers are replaced by passive interfaces described earlier. Each ofthe plurality of electronic receivers is then electronically connectedto one of a plurality of electronic analog-to-digital converters so thateach electronic analog-to-digital converter only sees pulses at itselectronic sampling rate. The electronic sampling rate of eachanalog-to-digital converter is many times slower than the opticalsampling rate. A digital back end process unit is then used for variousprocessing tasks. The optical pulse power is chosen to be large enoughso that the equivalent input noise of the photonic ADC (at the RF input610 shown in FIG. 6, for example) is smaller than the equivalent inputnoise at the input of the electronic ADCs (at the inputs to the ADCs 630shown in FIG. 6, for example). This is possible because, as the opticalpower increases, the variation in the pulse amplitude at the output ofthe receivers 626 caused by a varying voltage at the photonic ADC input610 to become larger. When the output pulse amplitude variation becomeslarger than the input voltage variation of the signal, there is gain andthe equivalent input noise at the photonic ADC input becomes smallerthan the equivalent input noise of the electronic ADC. This simpledescription assumes the equivalent input noise of the photonic ADC isdominated by the equivalent input noise of the electronic ADC. This ispossible by careful design of the photonic front end to minimizephotonic noise sources, such as laser intensity noise and opticalamplifier noise. Techniques for this are well known; see, for example,E. I. Ackerman, et al, “Signal-to-noise performance of two analogphotonic links using different noise reduction techniques,” 2007International Microwave Symposium Conference Digest, pp. 51-54, Jun.3-8, 2007.

Equivalents

While the Applicant's teaching is described in conjunction with variousembodiments, it is not intended that the Applicant's teaching be limitedto such embodiments. On the contrary, the Applicant's teaching encompassvarious alternatives, modifications, and equivalents, as will beappreciated by those of skill in the art, which may be made thereinwithout departing from the spirit and scope of the teaching.

1-49. (canceled)
 50. A photonically-sampled electronically-quantizedanalog-to-digital converter comprising: a) an optical transmitter thatgenerates an optical signal comprising a series of optical pulses; b) anoptical modulator having an input that is optically coupled to an outputof the optical transmitter, an RF input that receives an input RFmodulation signal, and an output that provides a modulated opticalsignal, wherein the optical pulses in the optical signal comprising aseries of optical pulses sample the input RF modulation signal togenerate an RF-modulated optically-sampled signal at an output; c) anoptical pulse demultiplexer having an input that is coupled to an outputof the optical modulator, the optical pulse demultiplexer generating aplurality of demultiplexed RF-modulated optically-sampled signals at aplurality of outputs from the RF-modulated optically-sampled signal; d)a plurality of optical detectors, an input of a respective one of theplurality of optical detectors being optically coupled to an output of arespective one of the plurality of outputs of the optical pulsedemultiplexer, an output of each of the plurality of optical detectorsbeing electrically connected to an input of a respective one of aplurality of analog-to-digital converters; and e) a digital signalprocessor having a plurality of inputs electrically connected to anoutput of a respective one of the plurality of analog-to-digitalconverters, the digital signal processor generating a digitalrepresentation of the input RF signal.
 51. The photonically-sampledelectronically-quantized analog-to-digital converter of claim 50 furthercomprising a dispersive optical element having an input that isoptically coupled to the output of the optical modulator, the dispersiveoptical element increasing a width of the optical pulses of themodulated optical signal.
 52. The photonically-sampledelectronically-quantized analog-to-digital converter of claim 51 whereinthe dispersive optical element is configured so as to provide a desiredpulse length.
 53. The photonically-sampled electronically-quantizedanalog-to-digital converter of claim 51 wherein the dispersive opticalelement is configured to lower a noise of the photonically-sampledelectronically-quantized analog-to-digital converter.
 54. Thephotonically-sampled electronically-quantized analog-to-digitalconverter of claim 51 wherein the dispersive optical element isconfigured to lower a power of the optical pulses to a level thatreduces nonlinearities in the plurality of optical detectors.
 55. Thephotonically-sampled electronically-quantized analog-to-digitalconverter of claim 51 wherein the dispersive optical element comprises alength of optical fiber.
 56. The photonically-sampledelectronically-quantized analog-to-digital converter of claim 50 whereinthe series of optical pulses comprises a series of substantiallyequal-amplitude optical pulses.
 57. The photonically-sampledelectronically-quantized analog-to-digital converter of claim 50 whereinthe optical transmitter generates the series of optical pulses withuniform spacing.
 58. The photonically-sampled electronically-quantizedanalog-to-digital converter of claim 50 wherein the optical transmittergenerates the series of optical pulses with non-uniform spacing.
 59. Thephotonically-sampled electronically-quantized analog-to-digitalconverter of claim 50 wherein each of the plurality of demultiplexedRF-modulated optically-sampled signals comprises a time slot of theoptical signal.
 60. The photonically-sampled electronically-quantizedanalog-to-digital converter of claim 50 wherein the digital signalprocessor is configured to perform a combining function.
 61. Thephotonically-sampled electronically-quantized analog-to-digitalconverter of claim 50 wherein the digital signal processor is configuredto perform a digital noise cancellation function.
 62. Thephotonically-sampled electronically-quantized analog-to-digitalconverter of claim 50 wherein the digital signal processor is configuredto perform a calibration function.
 63. The photonically-sampledelectronically-quantized analog-to-digital converter of claim 50 whereinthe digital signal processor is configured to perform a linearizationfunction.
 64. The photonically-sampled electronically-quantizedanalog-to-digital converter of claim 50 further comprising a pluralityof dispersive optical elements each of the plurality of dispersiveoptical elements having an input that is optically coupled to arespective one of the plurality of outputs of the optical pulsedemultiplexer, the plurality of dispersive optical elements increasing awidth of the optical pulses in the demultiplexed RF-modulatedoptically-sampled signals at the respective one of the plurality ofoutputs.
 65. The photonically-sampled electronically-quantizedanalog-to-digital converter of claim 50 wherein an output of each of theplurality of optical detectors is electrically connected to an input ofa respective one of a plurality of passive electrical interfaces, andoutput of each of the plurality of passive electrical interfaces iselectrically connected to an input of a respective one of a plurality ofsignal analog-to-digital converters.
 66. The photonically-sampledelectronically-quantized analog-to-digital converter of claim 65 whereinthe outputs of the plurality of passive electrical interfaces drive therespective ones of the inputs of the plurality of signalanalog-to-digital converters with a current that achieves greater thanhalf-scale.
 67. The photonically-sampled electronically-quantizedanalog-to-digital converter of claim 65 wherein the outputs of theplurality of passive electrical interfaces drive the respective ones ofthe plurality of inputs of the signal analog-to-digital converters witha current that achieves substantially full scale.
 68. Thephotonically-sampled electronically-quantized analog-to-digitalconverter of claim 50 wherein the modulator comprises a complementaryoutput optical modulator having an input that is optically coupled to anoutput of the optical transmitter and an RF input that receives an inputRF modulation signal and that generates an RF-modulatedoptically-sampled signal at a positive output that is coupled to aninput of the optical pulse demultiplexer that generates a plurality ofdemultiplexed RF-modulated optically-sampled signals at a plurality ofoutputs and a negative output that is coupled to an input of a secondoptical pulse demultiplexer that generates a plurality of demultiplexedRF-modulated optically-sampled signals at a plurality of outputs. 69.The photonically-sampled electronically-quantized analog-to-digitalconverter of claim 68 further comprising a second plurality of opticaldetectors, an input of a respective one of the second plurality ofoptical detectors being optically coupled to an output of a respectiveone of the plurality of outputs of the second optical pulsedemultiplexer.